New study says 30 minutes of exercise a day is not enough. You should double or quadruple that.

If you're among of the millions of Americans who dutifully carve out 30 minutes a day for the moderate-intensity exercise recommended by experts based on the idea that you're doing all you can for your heart, you're in for some disappointing news.

A new analysis published Monday in the journal Circulation finds that that amount of activity may not be good enough.

For the paper, researchers reviewed 12 studies involving 370,460 men and women with varying levels of physical activity. Over a mean follow-up time of 15 years, this group experienced 20,203 heart failure events. Each of the participants self-reported their daily activities, allowing the team to estimate the amount of exercise they were doing.

They found that those following the 30-minutes-a-day guidelines issued by the American Heart Association had “modest reductions” in heart failure risk compared to those who did not work out at all.

But those who exercised twice and four times as much had “a substantial risk reduction" of 20 percent and 35 percent, respectively.

The findings challenge the notion of a 30-minutes-a-day magic number for exercise. Instead, research found that physical activity and heart failure may be what they called "dose dependent," meaning that higher levels of physical activity appeared to be linked to a lower risk of heart failure. That association appeared to hold across age groups, gender and race.

Jarett D. Berry, senior author of the study and an associate professor at University of Texas Southwestern Medical School, said the study shows that physicians and health policymakers should consider making stronger recommendations for greater amounts of physical activity to prevent heart failure.

Katie Ledecky of the United States swims in the women's 200-meter freestyle final at the Aquatics World Championships in Kazan, Russia, in August. (Michael Dalder/Reuters)

Heart failure, which occurs when the heart cannot supply enough blood to the body, affects more than 5.1 million adults in the country, and results in health-care costs exceeding $30 billion per year. The American Heart Association reports that it accounts for “a significant proportion” of hospitalizations and deaths of older Americans. Officials suggest that this “growing epidemic” is expected to increase by 25 percent from 2010 to 2030.

“Heart failure is a big public health concern and in contrast to the dramatic reduction in coronary disease that we’ve seen in the population, the incidence of heart failure remains relatively unchanged,” Berry said.

American Heart Association guidelines recommend that middle-aged adults engage in at least two hours and 30 minutes per week of exercise such as brisk walking. Berry said walking 30 minutes a day, for instance, may not be enough for a middle-aged person with hypertension, which presents an increased risk of developing heart failure. Those with diabetes or a history of heart failure also would benefit from talking with their doctors about increased physical activity.

Ambarish Pandey, the study’s lead author and a cardiology fellow at the University of Texas Southwestern Medical School, said the study was limited in its ability to compare the relationship of heart failure risk with different types of physical activity, as well as in differentiating between work-related physical activity vs. exercise for leisure.

“If someone runs to their work, that doesn’t count as leisure,” Pandey said. “That counts as occupational. If someone is an exercise trainer, then he will be more active at his workplace and that may not be accounted for in the leisure activity that we have looked at.”

If there's no way you can cram in 300 to 600 minutes of exercise a week, don't despair. Plenty of research shows that lower amounts, even microbursts of intense 10 to 15 minutes of activity, can be beneficial.

In fact, the Centers for Disease Control and Prevention notes that one minute of vigorous activity is about the same as two minutes of moderate activity, so you can do a mix of the two each week.

Vigorous aerobic activity is defined as a something that makes you breathe hard and fast and that pushes up your heart rate pretty high. "If you're working at this level, you won't be able to say more than a few words without pausing for a breath," government guidelines note. Here are some examples:

Jogging or running

Swimming laps

Riding a bike fast or on hills

Playing singles tennis

Playing basketball

Build up over time

As for moderate-intensity exercise, there are plenty of activities you may not have thought of that can help your heart and "count" toward your exercise total. Examples include:

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